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Microscope objectives in photomicrography

Doug Kerr

Well-known member
Caveat: I write here about a topic about which I know just enough to be dangerous. I certainly invite those here with real knowledge of the matter to jump in and straighten me out.

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We often read (as for example in the current Popular Photography article on the amazing insect photomicrography of John Hallmén) something like this (the color emphasis is mine, intended to facilitate reference later):

"Modern microscope objectives are focused at infinity, and to compensate for that we must use something like a morfanon tube in our photomicrography setup."​

I find the blue passage curious, since a lens (including a microscope objective lens) cannot itself be focused (that is, onto an object) at any distance; only the instrument of which it is a part (camera, microscope) can be focused at a certain distance. In fact the blue passage is just not accurate. What would be more accurate is this:

"Modern microscope objectives are designed so that the various corrections for aberrations are optimal when the lens is focused on an object such that the image is produced at "behind the lens infinity". To accommodate that arrangement, we must use something like a morfanon tube in our photomicrography setup."​

To get an insight into the significance of this, we need to look into some history of a certain aspect of microscope development.

Until perhaps ten years ago, microscope objective lenses were designed so that the various corrections for aberrations are optimal when the lens is focused on an object such that the image is produced at a finite distance behind the lens (not too far in front of the eyepiece lens). The eyepiece lens than picks up that real aerial image and relays it to the user's eye.

Of course, for this to work out when different objectives were in place, the objectives all had to be set up so that their corrections would be optimal when they were focused on an object such that the image was produced at a consistent location in the microscope tube. That distance was eventually standardized by the microscope industry at 160 mm behind the objective lens.

It turns out (and here I am getting way beyond my own knowledge of microscopy) that we often want to introduce auxiliary elements behind the objective. Doing so disrupts the correction of the objective.

It turns out that if the objective were designed to that when it was focused on an object at the optimal distance (with respect to the aberration corrections of the lens) the image was formed at "infinity behind the lens", those auxiliary elements could be introduced with disruption to the objective correction. (No, I don't understand how that works.)

But of course now the image produced by the objective is not suitable for further handling by the eyepiece lens, which expects to find the image produced by the objective just in front of it, not an infinite distance behind it.

So in fact, in modern microscopes, there partway up in the tube a "relay lens" which takes the parallel rays emerging from the objective (which would form at image at "rear infinity") and converges them to in fact generate an aerial image just in front of the eyepiece lens.

There is sufficient distance between the objective and this relay lens (this region is called the "infinity space") to insert these auxiliary elements, which there (where the rays from the objective are parallel) will cause the least disruption to the correction of the objective.

That relay lens, located partway up in the microscope tube, is called the "tube lens".

And of course such microscopes use objectives whose aberration corrections are optimal when the lens is focused on an object such that the image is produced at "rear infinity".

Now when we use this type of microscope objective on a camera, it is not suitable to the camera proper to have the objective produce its image at "rear infinity". The camera cannot focus there.

I don't understand that, as a real image at "rear infinity" is a virtual image at "front infinity, and it seems as if the camera with its regular lens should be able to focus there, but of course there is much about this matter I don't understand.​

So we must, just as in a microscope, have a "tube lens" in the system. And that tube lens is located in an accessory, a special type of "extension tube" placed between the microscope objective and the camera's prime lens. (It also takes care of adaptation to the mounting of the microscope objective.) And this, for a reason I don't yet know, is called a "morfanon tube".

Best regards,

Doug
 

Doug Kerr

Well-known member
Hi, Klaus,

a simple 200mm Tele works quite well actually and an adapter plate that allows the Objective to be screwed in front of that lens (set at infinity of course).

RAYNOX 250 achromatic diopter also works well for that.

Good to know.


Thank you for that reference. I think I encountered that article when doing the research for my note but didn't actually read it. I see now that it has a lot of very useful information about this topic (about which I am of course at best a dilettante)!

Thanks again.

Best regards,

Doug
 
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